Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and flash memory.
Flash memory devices have developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Common uses for flash memory include personal computers, personal digital assistants (PDAs), digital cameras, and cellular telephones. Program code and system data, such as a basic input/output system (BIOS), are typically stored in flash memory devices for use in personal computer systems.
As the performance and complexity of electronic systems increase, the requirement for additional memory in a system also increases. However, in order to continue to reduce the costs of the system, the parts count must be kept to a minimum. This can be accomplished by increasing the memory density of an integrated circuit by using such technologies as multilevel cells (MLC). For example, MLC NAND flash memory is a cost effective non-volatile memory.
Multilevel cells take advantage of the analog nature of a traditional flash cell by assigning a bit pattern to a specific threshold voltage (Vt) range stored on the cell. This technology permits the storage of two or more bits per cell, depending on the quantity of voltage ranges assigned to the cell and the stability of the assigned voltage ranges during the lifetime operation of the memory cell.
For example, a cell may be assigned four different voltage ranges of 200 mV for each range. Typically, a dead space or margin of 0.2V to 0.4V is between each range to keep the Vt distributions from overlapping. If the voltage stored on the cell is within the first range, the cell is storing a logical 11 state and is typically considered the erased state of the cell. If the voltage is within the second range, the cell is storing a logical 01 state. This continues for as many ranges that are used for the cell provided these voltage ranges remain stable during the lifetime operation of the memory cell.
For MLC NAND flash memory devices, two or more pages of data may be written into memory cells corresponding to a single physical page. Moreover, for some MLC NAND flash memory devices, the order in which data is written to rows of a memory array may be scrambled, since such scrambling can act to mitigate disturb issues. For example, logical page 0 may get written to a physical lower page, and logical page 4 may get written to the corresponding physical upper page. Therefore, one problem involving MLC NAND flash memory devices is that if a program operation is unsuccessful, not only the present upper page gets corrupted, but the lower companion page may also get corrupted. Moreover, the scrambling mechanism typically varies from designer to designer and is generally not accessible to the end user. Therefore, only the designer may know what page corresponds to the companion page.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for alternatives to existing programming schemes for MLC NAND flash memory devices.